1. Field of the Invention
The invention relates generally to sol-gel glasses and a chemical procedure for their preparation. Preferred "crack-free" glasses are polymerized from substituted metal alkoxides having two or three hydrolyzable groups. Thin films up to 100 .mu. are typically obtained in a rapid single step process. The invention also relates to an accelerated curing method for condensation polymers by glow discharge over the polymer bulk.
2. Description of Related Art
Sol-gel techniques have been extensively investigated for more than two decades (Gottardi, 1982) and used to prepare glasses and ceramics for use in a wide variety of applications, employing various precursors, catalysts, additives and procedures. Sol-gel precursors most investigated have been prepared from siloxanes, especially tetraalkoxysilanes (Reisfeld, 1987). Titania, alumina (Kobayashi, 1988) and mixed glasses have also been investigated. Numerous chromophores have been incorporated into sol-gel produced glassy materials, laser-dyes in particular. The latter have exhibited promising characteristics for use in nonlinear optics (NLO), especially for laser systems (Reisfeld, 1989). Dye-embodying supported glass thin films have been prepared by sol-gel techniques, aiming at surface laser systems (Kobayashi, 1988), yet prolonged and complex processes are required to facilitate crack-free glasses of the desired properties.
Tetraethoxysilane is the favored precursor for the preparations of pure silica (SiO.sub.2) glasses by the sol-gel method due to its moderate reaction rate. The water-to-siloxane molar ratios (MR) most commonly employed is 5:1 to 10:1. A co-solvent (e.g. ethanol) is regularly added to maintain a one-phase reaction solution, although it was recently demonstrated, in the sol-gel reaction of tetramethoxysilane, that the methanol produced by the hydrolysis was sufficient to maintain a single-phase at an early stage of the reaction: EQU Si(OMe).sub.4 +H.sub.2 O .fwdarw.Si(OMe).sub.3 OH+MeOH
Following the partial or complete hydrolysis of the silicon-alkoxide precursor, polycondensation of the hydroxyl groups takes place via the si-O-si etheric bonds. The glassy matrix formed by this polymerization is capable of encaging large molecules (e.g. chromophores, enzymes) which have been introduced into the reaction mixture. However, during this stage of the sol-gel synthesis severe cracking and fragmentation of the formed glass are the common obstacles that impede the fabrication of articles and films in general, and supported films in particular. This is due to the extensive volume-contraction which accompanies the condensation reactions and the corresponding expulsion of the solvent and the condensation products. Many investigators have attempted to overcome this obstacle by using a wide variety of additives such as dimethylformamide (DMF), formamide, organic acids or surfactants. Even with these additives, however, an extremely slow and very cautious drying of the sol-gel glass is necessary for the survival of a fracture-free glass, making this synthetic route more of an art than a science.
The cracking problem is further aggravated where sol-gel glasses are cast onto a rigid support since the gelling matrix is no longer free to contract. Crack-free supported glass-films can be maintained by a surfactant, yet a high concentration (up to 3% in the final glass) is required. Its durability towards decomposition under conditions of high energy-density (e.g. solid-state laser) is questionable.
Although crack-free glasses from sol-gels have been prepared, the procedures are time-consuming and often complex. Of particular concern from a practical aspect is the undue amount of time required in curing glass thin-films on supported matrices. It is not unusual to dry polysiloxane films for periods of weeks to assure a quality glass. Such films, depending on conditions of drying, additives and process modifications have required from 6 to 45 days for complete drying to crack-free films (Hara, 1991).